To the south-east of Portus are these two stretches of the city wall. They begin at city gate 20 and end at the Fossa Traiana.

The wall follows the north side of a curved channel, dug by order of Trajan, that connected the Fossa to the south-west and the Tiber to the north-east. It was c. 35 metres wide and originally it was flanked by towpaths.